COVID-19 Testing

Testing should be targeted and purposeful NOT random and irrational. Presently there are very confused messages regarding testing, made worse by profiteering based on fear and ignorance.

Facts about Testing

  1.  No test is 100% accurate i.e. perfect – don’t believe anything to the contrary.
  2. All tests have limitations/pros and cons.
  3. The ‘accuracy’ of a test depends on many factors, not just one:
    a.     The reliability of the sample taken (which depends on who takes it).
    b.     The ‘chain of custody’ of the sample i.e. its journey from the sample taken at source, to the point of analysis of the sample, to the result being provided to the person sampled.
    c.     The tests ‘accuracy’, this is often quoted in terms of ‘specificity’ and ‘sensitivity’
    i.     Specificity is the ability of a test to provide true negatives i.e. a negative result when you don’t have the virus.
    ii.     Sensitivity is the ability of a test to provide true positives i.e. a positive result when you do have the virus.

    Many tests for Covid-19 are highly specific e.g. 99.8%. However, the sensitivity of a test depends on the prevalence of the hazard you are testing i.e. how many cases there are in a population at a given time. If the prevalence is high e.g. 5% the test may be highly sensitive, however if it is low e.g. 0.5% it may be far less sensitive. The problem is we don’t know the true prevalence of the disease, we only know the incidence i.e. the number of new cases.

What’s the difference between the tests?

 Generally there are two types of test at present:

A molecular/ swab (antigen) test.

This is based on taking a oral swab which will be sent to a laboratory where it is assessed by a Polymerase Chain Reaction (PCR) which identifies the virus’s RNA. As such the test is either positive or negative. If negative you could still become infected at a later date.

Pros:

  • The sample is analysed by a laboratory, which should be more accurate. However, laboratories have made errors and samples have been sent to alternative laboratories for testing.
  • If the sample is collected by an appropriately trained healthcare professional it should be of a high quality.

Cons:

  • Chain of Custody – the sample can get lost or misidentified during it’s journey. It is also exposed to environmental factors such as heat, light and humidity over time, which might degrade the sample.
  • If sampling is conducted by someone other than a healthcare professional it could be of poor quality i.e. in the cases of tests/kits sent to homes, or where individuals are asked to self-collect.

Serology (antibody) test.

Presently, antibody test kits come in two versions:

1. Those which can give an immediate result but are only licensed for use by healthcare professionals who perform the sampling and interpret the results e.g. doctors.


These are based on taking a sample of blood (usually only a pin-prick sample) dispensing it into a testing device, adding a buffer solution, and timing the result (usually 10-15 minutes). The test is devised to identify antibodies to the COVID-19 virus (IgM and IgG antibodies). If no antibodies are found the test is negative i.e. the person does not have the virus. If IgM antibodies are detected then they have the infection. If they have IgM and/or IgG antibodies they are in the later stages of the infection and ‘could’ be immune*1.

2. Those which can be conducted by an individual at home but must then be sent off to a laboratory for analysis.

 

Pros:

  • The test not only tests if an individual has been infected or not, it tests for anitbodies, which could indicate a level of immunity.
  • The test, if conducted by a healthcare professional, will provide a good sample (if not it can be repeated) and the results are available immediately. They are interpreted by the healthcare professional who can advise with regard to the result and any further actions the individual should take e.g. with regard to work (this would not be the case if the sample was taken by the individual themselves).
  • If the test is conducted by a healthcare professional there should be no issues with regard to chain of custody, since from the time of sampling to obtaining the results there is no break in the chain i.e. the sample never leaves the sight of the healthcare professional. This maintains the integrity of the sample (this would not be the case if the sample was taken by the individual themselves because they would need to send it to a laboratory).
  • Since the test is completed within 10-15 minutes the sample is not exposed to environmental hazards over time, such as temperature, light or humidity.

Cons:

  • In theory a laboratory test ‘should’ be more accurate than a Point of Care (POC) test. However, one still needs to factor in the other reasons for reduced accuracy in a test e.g. sampling, chain of custody, type of laboratory, etc.
  • For the best result the individual should be tested 2-4 weeks after the cessation of symptoms, although in many cases the individual may have never had any symptoms. Generally, testing should not be performed on those who are symptomatic i.e. presently have flu-like symptoms e.g. dry cough, fever, etc. However, If the test is to be conducted by a healthcare professional a questionnaire can form part of the consultation and therefore this entire issue can be avoided.

*1 Dr. Maria Van Kerkhove of The World Health Organisation made the point on 25th April 2020 that there was no evidence that people who had been tested positive were “necessarily immune” and could not acquire the infection again. Clearly there are no guarantees regarding either immunity or secondary infection. However, the presence of antibodies are consistent with immunity, and the chances of secondary infection are probably low (except in vulnerable groups). In either case this, in our opinion, sent out a rather confusing message. It might have been better to have said a positive IgM/G test shows you have formed antibodies to an infection by COVID-19.

Interestingly, the WHO made something of a U-turn on that comment by later adding, “We expect that most people who are infected with Covid-19 will develop an antibody response that will provide some level of protection.”

 Our conclusion is that whatever test is used it should be within the framework of a consultation with an appropiately trained healthcare professional who understands the issues and can explain and advise individuals and organsiations. Importantly such a healthcare professional can ensure the individuals and organisations understand exactly what a test is and is not, it’s benefits, limitations, and risks. All testing does not replace the need for a workplace assessment and implementation of control measures. Therefore we offer testing on that basis. We think that's simply 'good medicine'.

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